This invention relates to a process for the oxygen bleaching of wood or non-wood pulp using an agglomerated bleach activator to react with hydrogen peroxide to form a pulp bleaching solutions. In particular it relates to the formation of a pulp bleaching solution by the reaction of a specially formulated agglomerate of a crystalline powder of Tetraacetyl ethylene diamine, commonly known as TAED, with excess hydrogen peroxide and the use of that bleaching solution to bleach and delignify pulp.
Oxygen based bleaching is used for pulp bleaching bee of its environmental benefits and oxidising power. Hydrogen peroxide is increasingly being used in oxygen based pulp bleaching.
Although hydrogen peroxide has environmental benefits over chlorine based bleaches it suffers from some performance disadvantages which become particularly important when an existing pulp bleaching sequence is being converted to operate with hydrogen peroxide. One problem is that the degree of whiteness may be reduced compared with the use of chlorine dioxide, this may significantly reduce the value of the pulp.
A second problem which may occur when process conditions are changed to optimise bleaching for use of hydrogen peroxide is that the fibre length deteriorates and there is a corresponding loss of strength in paper produced from the pulp. A third potential problem is the poor delignification performance of hydrogen peroxide when used on its own.
It has been suggested that use of a stronger oxygen based bleach will solve these problems. However, whilst use of equilibrium peracetic acid or Caro""s acid improves the bleaching and reduces colour reversion it also introduces problems of handling and dosing peracid in the pulp plant. It is proposed in several patent applications to react at least part of the hydrogen peroxide with a bleach activator thereby generating a peroxyacid based oxygen bleaching species in situ which facilitates improved pulp bleaching and delignification whilst avoiding the hazards of transporting preformed peracid or generating peracid by reaction of hydrogen peroxide and carboxylic acid under forcing conditions. In WO9521290 there is described a process in which peracid is produced in siti by reaction of a bleach activator such as tetraacetylethylenediamine and hydrogen peroxide at a pH less than the pKa of peracetic acid formed from the reaction of these chemicals. It is stated that in a preferred process the TAED is first dissolved in hot water and then added to the hydrogen peroxide before the reacting mixture is dosed to the pulp. Sequestrant may be added to the pulp before the dosing takes place. It is also stated that conditions must be optimized to ensure that all the TAED is consumed. The chemistry must be carefully controlled to achieve consistent results when using such a powerful bleach as peracetic acid.
In EP456032 there is described a similar pulp bleaching process using alkaline TAED and hydrogen peroxide. Bleaching of the pulp is done in plastic bags and no detail is given of how a scaled up process should be operated.
JP05186989 proposes an alkaline process using a bleach activator such as TAED to give brighter pulp when it is used with oxygen and optionally also with hydrogen peroxide, very few details of the process steps are given and hydrogen peroxide is not used in the examples. It appears that the TAED is mixed as a solid with dry pulp at the start of the bleaching step.
GB2304126 proposes use of TAED activated hydrogen peroxide to delignify pulp. Again the form of the TAED is not specified.
It is conventional in pulp bleaching to use letters to represent stages in a process. Some of the commonly used letters are: C for chlorine, D for chlorine dioxide, Z for ozone, P for peroxide, Pa for peracetic acid, O for oxygen, Q for chelation, and Px for mixed peracids. In this specification the notation PA is used for the combined addition of a bleaching solution containing hydrogen peroxide and a peracid formed by reaction of the hydrogen peroxide with an agglomerated bleach activator.
TAED is produced as a fine white crystalline powder. Its main use is as a bleach activator for perborate and percarbonate salts in granulated laundry detergent formulations. TAED is normally used in a laundry detergent formulation which contains a large amount of anionic surfactant. In this environment the main problem facing the skilled person is to keep water away from the TAED during storage. To give increased stability and to prevent segregation due to size differences the TAED is therefore agglomerated using one or more binders. A commonly used binder is Sodium Carboxymethylcellulose. The agglomerate typically contains from 80 to about 90% TAED and the remainder is made up of the binder system and optional disintegration and dispersion aids together with other minor components such as pigments, colorants and sequestrants. Sometimes it is beneficial to utilise a co-binder as specified in EP0764717. TAED has also been proposed to be formed into noodles by extrusion. GB1395006 teaches formation of such noodles using 25% by weight of an anionic surfactant mixture including Sodium Laurylsulphate as the binder. Nonionic surfactants with a melting point greater than 35xc2x0 C. have also been proposed as binders, they are usually heated during the manufacture e.g. using the spray-melt process of DE 2406455. In practice the remainder of the detergent formulation is critical to the successful dispersion and dissolution of the activator as it provides the necessary pH control and surfactants.
WO9418298 describes a bleaching process where an N-acyl bleach activator is reacted with a source of hydrogen peroxide under acid conditions. The product of this reaction may be used in many bleaching and disinfection applications, including pulp and paper bleaching. The activator and other components may be in the form of particles and these particles may be provided by techniques similar to those used in the laundry detergent industry For instance by spray drying liquid slurries; by granulation techniques using binders, for instance synthetic or natural polymers (or derivatives); or by melt blending followed by extrusion or other techniques. A composite product including a bleach activator may also include other additives, especially heavy metal sequestrants and it may include surfactants to act as wetting agents and inorganic salts to act as a diluent or to increase the rate of disintegration or dissolution of the product. The composite product should also include the source of the hydrogen peroxide as well as the bleach activator when it includes the wetting agent. Only two granulated activator particles are exemplified in this document; both contain carboxymethyl cellulose as a binder and neither is used for pulp bleaching.
WO9725402 proposes the use of bleach activators such as TAED for various applications including pulp bleaching. The preferred form of the TAED is a granule, but no details are given of the composition of the granule.
WO9517497 describes a process for preparing a particulate composition containing a liquid bleach activator, a thickener which is preferred to be a polyhydroxy fatty acid amide surfactant and optional filler and non-amide solubilising surfactants. The optional solubilising surfactants may be anionic especially C11-13 linear alkylbenzenesulphonates, the optional particulate filler material is selected from a group of materials which includes: sodium acetate, sodium phosphate, sodium acid phosphite and sodium sulphate, however, all the examples use sodium alumino-silicate.
WO9800504 discloses the use of an anionic surfactant as part of the binder system for a bleach activator agglomerate. A wide range of bleach precursors are suggested to be suitable, including TAED. Formulation 4 of Example 1 is a bleach precursor particulate containing 65% TAED, 9.8% sodium linear C12 alkyl benzenesulphonate (anionic surfactant), 0.3% C2-4predominantly linear primary alcohol condensed with an average of 3 moles of ethylene oxide (nonionic surfactant), 0.96% sodium toluene sulphonate, 11.3% citric acid and 6.2% sodium carboxymethylcellulose, the balance (nearly 6.5%) is water. The only other example using TAED also contains 6.2% sodium carboxymethylcellulose and again contains over 6% water.
We have found that use of TAED powder in a pre-reaction with hydrogen peroxide to form a bleaching solution for use in a pulp bleaching process is undesirable for a number of reasons. Firstly it does not have the required free flow properties to be dosed effectively from a hopper. Secondly it requires special equipment to control dust levels. Thirdly it does not mix well with hydrogen peroxide solution and has poor dispersing and dissolution characteristics. These disadvantages can be overcome by use of an agglomerated product which enables the use of small crystal size TAED without the associated problems of dusting, high risks, poor stability and poor solids handling. These problems and their solution are different from those encountered in the laundry detergent industry. In that case the granulate is not fed from a bulk storage hopper to a reaction vessel, neither is the reaction carried out with so few charged species in solution. For instance a typical laundry detergent containing TAED will also contain sodium perborate, sodium silicate and a sodium salt of anionic surfactant.
Thus we have found that use of agglomerates which are designed for laundry detergent applications leads to several unforeseen problems in a pulp bleaching process. Firstly many of the binders used in laundry applications are insoluble under the pH and temperature conditions under which an agglomerate may need to be pre-reacted with the hydrogen peroxide solution. Secondly, many binders react with the caustic soda added to adjust the alkalinity of the pre-reaction mixture and/or the pulp. This reaction or complexing can form a scum, which is unacceptable for pulp bleaching where residual solid matter must be avoided. Thirdly the binders may give rise to poor pulp bleaching and may even form coloured species under the extremes of one or more of temperature, pressure and residence time encountered in pulp some bleaching processes. Fourthly, the agglomerates made with conventional binder systems to not disperse fast enough or wet fast enough in the low ionic medium of the pre-reaction process.
Following the efforts of the present inventors a range of binders and dispersing systems has been selected which are suited to a process which reacts with TAED with dilute hydrogen peroxide on a large scale for continuous dosing to a pulp bleaching process.
According to the present invention there is provided a process for the bleaching of wood and non-wood pulp comprising the steps of:
a) adding an aliquot of agglomerate from a bulk solids dispersing means, advantageously a hopper, to a dilute solution of hydrogen peroxide which is desirably agitated to keep the agglomerate in suspension, the agglomerate comprising:
a bleach activator, advantageously TAED;
a peroxide soluble binder, preferably polyvinyl alcohol (PVOH);
a dispersing system comprising a wetting agent which is a low foaming and does not discolour on exposure to temperatures of up to 120xc2x0 C. and optionally a salt, preferably sodium acetate, which is highly soluble in hydrogen peroxide;
and sequestrant which may additionally or alternatively be added directly to the hydrogen peroxide;
b) allowing the TAED to substantially completely react with the hydrogen peroxide;
c) adjusting the pH of the bleach solution by adding alkali preferably caustic soda; and
d) bleaching the pulp by contacting it with the bleaching solution under alkaline conditions.
The agglomerate preferably comprises: a TAED agglomerate with average particle size in the range 5 to 2000 micron having a binder system comprising less than 1%, preferably less than 0.5% CMC, most preferably none at all and: 2-8% of anionic surfactant which can be dried to a solid and other ingredients which are non precipitating over a pH range of 5-10, the binder system further being completely compatible with the pulp bleaching process and preferably readily biodegradable.
Surprisingly, although pulp is mainly cellulosic material, we have discovered that the use of a cellulosic binder material is to be avoided because it reacts with other ingredients to form an insoluble scum which cannot be removed during the bleaching process, this is critically important and means that a conventional laundry detergent bleach activator granule such as that disclosed in EP37026 cannot be used for pulp bleaching.
Preferably the anionic surfactant is incorporated at a level of 1 to 6% by weight based on the dry agglomerate, most preferably about 2.5%.
For processing reasons the agglomerate may comprise up to 1% preferably 0.1 to 0.5% by weight based on the dry agglomeration of a co-binder, such as Sodium Carboxymethylcellulose. However, an agglomerate without any CMC is preferred.
Other additives such as flow aids, sequestrants, pH adjusting components, diluents and the like may also be included in the agglomerate as required. The inclusion of one or more sequestrants is particularly advantageous as these are needed to enable any transition metals in the alkali used yo adjust the pH to be rendered non-catalytic for the decomposition of the hydrogen peroxide. If they are not added as part of the agglomerate they need to be added separately to the pre-reaction vessel.
The use of sodium acetate as the salt is preferred because the reaction products of TAED and peroxide may in any case form sodium acetate at neutral pH so the use of this salt does not add to the chemical complexity of the system This can be important for waste treatment. Sodium acetate also has the advantage that the segregation of the agglomerate during transportation and storage is low.
The agglomerates may be manufactured using any process known to those skilled in the art e.g. mixing TAED powder and a solution of surfactant to form agglomerates and drying the agglomerates so formed.
The bleaching solution comprises an oxygen bleach mixture which is advantageously formed by the reaction of the bleach activator dispersed from the agglomerate with a molar excess of hydrogen peroxide over the acetyl groups that are released from the bleach activator, typically a 10:1 excess is used. This provides an advantage over the use of preformed peracid because it eliminates the need to handle the peracid in concentrated form. The pH of the bleaching solution is adjusted by addition of alkali before it is dosed to the pulp.
The preferred bleach activators are solid as this enables the agglomerate to be accurately dosed and allows for easy recovery from accidental spillages.
The peroxide source is hydrogen peroxide. The concentration of hydrogen peroxide in the pre-reaction mixture is normally in the range 0.1 to 60%, preferably 0.2 to 30% w/v. A typical level will be 3%. The amount of bleach activator used in the prereaction mixture should be in the range 0.001 to 20 g/l based on the theoretical dose to dry pulp, although it has been found in practice that production of the reaction product in this way is not very ratio sensitive. When TAED is used as the bleach activator we have found that use of large concentrations of TAED leads to an undesirable exothermic reaction and the maximum concentration that should be used is 10%, preferably 5% and most preferably less than 2%.
In the bleaching tower there should always be an excess of peroxide so that the peracetic acid bleaching complements the bleaching done by the hydrogen peroxide. This is done because the different bleaching species are able to bleach in slightly different ways which gives a synergistic increase in bleaching compared with that which would be obtained from the use of either chemical on its own. Furthermore the powerful nature of the peracid bleach means that a 4:1 molar excess of hydrogen peroxide over peracetic acid gives a reasonable balance in the bleaching contribution of the two components. The initial level of peroxide in the bleaching tower should be in the range 0.5 to 4% depending on the type of pulp and the process being used. The initial level of peracid in the tower will be equivalent to 0.5% bleach activator when TAED is used.
The pulp may be any sort of pulp, including chemical and mechanical pulp and mixtures thereof including recycled material. Wood and non-wood fibres can be bleached using this process. The flexibility of the process according to the invention allows the pH of the incoming oxygen bleach mixture to be adjusted to give the required pH in the pulp/bleach mixture although direct adjustment of the pH of the pulp is also acceptable.
The bleach activator may be any one or mixtures of more than one acetyl donor. Preferably, the activator is one or mixtures of more than one of the compounds of the formula I: 
in which L is a leaving group attached via an oxygen or a nitrogen atom to the Cxe2x95x90O carbon atom and R1 is acetyl.
The leaving group L is preferably a group, the conjugate add of which has a pKa in the range 4 to 13, preferably 7 to 11, most preferably 8 to 11.
Substituents on L can include hydroxyl, xe2x95x90Nxe2x80x94R2 in which R2 is preferably lower alkyl amine, acyl, acyloxy, alkoxy, aryl, aroyl, aryloxy, aroyloxy, halogen, amido, and imido groups and the like as well as other groups not adversely affecting the activity of the activator.
Activators generating peracids other than peracetic acid are either less effective or more costly and are not commercially useful.
Specific activators which are available for use in the invention are tetraacetylethylenediamine (TAED), pentaacetyl glucose (PAG), and tetraacetylglycoluril (TAGU). Of these TAED is preferred because it gives the most cost effective release of acetyl groups: it releases 2 such groups per molecule.
The addition of an alkaline salt such as caustic soda to the mixing stage will inevitably introduce transition metal ions. These are able to catalyse the decomposition of hydrogen peroxide so a sequestrant is used to prevent this. Preferred sequestrants are selected from the group comprising penta methylene phosphonic acid and diethylene triamine penta(methylene phosphonic acid) or DTPA.